You may have set something to open with something by default instead of something else.. You would have to find the app in the app settings, and clear the defaults on it.
For example,
Let's say you set all video files to open with a certain player but you really wanted to use the stock video player, you would have to go into that other player app settings, and clear the defaults. Then, next time you press on a video, options will pop up asking you which application to use to play the file.
This works the same way with the pictures. Your gallery somehow got set to open something else by default, and just needs to be cleared out. If for some reason this isn't your problem, then you'll probably just want to back up your pictures, music, videos, and do a reset on the phone.